Zinc isotope constraints on carbon cycle perturbations during the Cambrian radiation of animals

Increased oxygen and nutrient availability were likely crucial drivers of the Cambrian radiation of animals. However, productivity dynamics in the early Cambrian, and hence links to both metazoan radiation and global carbon cycle perturbations, remain poorly constrained. Zinc isotopes (δ66Zn) in marine sediments have emerged as a proxy for tracing productivity-induced perturbations to the carbon cycle. Here, we report δ66Zn data from two early Cambrian (ca. 522−514 Ma) siliciclastic successions on the Yangtze block, South China. Both sites show a major positive seawater δ66Zn excursion across the Cambrian Stage 2−3 boundary, synchronous with a global positive carbonate-carbon isotope (δ13Ccarb) shift. We interpret these concurrent excursions to reflect enhanced primary productivity and increased burial of isotopically light Zn in organic-rich sediments. Biogeochemical box modeling reproduces the observed δ66Zn−δ13Ccarb covariation, supporting coupled P-Zn-C cycling, in which elevated shallow-marine phosphorus (P) concentrations strengthened the biological pump and enhanced the removal of isotopically light Zn from seawater. Subsequently, reduced shallow-marine P concentrations and progressive ocean oxygenation collectively contributed to a decrease in seawater δ66Zn, either through reduced burial of isotopically light Zn in organic-rich sediments or the remobilization of previously buried isotopically light Zn. Together, our approach provides quantitative evidence that nutrient redistribution and productivity feedbacks governed early Cambrian Zn-C isotope dynamics, shedding light on the ecological backdrop of metazoan diversification.